<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .one {
            width: 100px;
            height: 50px;
            background-color: #00B0E7;
            border-radius: 10px;
            /*相对位置*/
            position: relative;
        }

        .one0 {
            width: 0;
            height: 0;
            border: 10px solid rgb(44, 31, 165);
            border-top: none;
            border-right-color: transparent;
            border-left-color: transparent;
            /*绝对位置*/
            position: absolute;
            /*将小三角形向下移90个像素*/
            top: -10px;
            /*将小三角形向右移10个像素*/
            left: 40px;

        }
    </style>
</head>
<body>
    ​​​​​​​<div class="one">
        <div class="one0"></div>
    </div>
</body>
</html>